The present invention relates to a method for evaluating kneaded clay used for manufacturing a ceramic structure used for a filter, a catalyst carrier, an electric transmission insulator, a refractory, or the like and to a method for manufacturing kneaded clay.
Ceramics, which are inorganic materials, have excellent physical, chemical, and electrical characteristics which metals and organic materials do not have. Further, ceramics have an advantage that any shape can be obtained from the raw material, and an example representing this is a honeycomb structure constituted of thin walls of some microns.
Generally, a ceramic structure is manufactured through a step of preparing kneaded clay from a mixed raw material containing a ceramic raw material, an organic binder, and/or an inorganic binder, a step of manufacturing a formed body by forming the kneaded clay into a desired shape, a step of drying the formed body, and the step of firing a formed article to obtain a fired article in this order.
The kneaded clay is prepared by mixing and kneading contained materials such as a ceramic raw material, a binder, and a solvent. The nature of the kneaded clay significantly influences on the characteristics of the ceramic structure. For example, when the kneaded clay is prepared with contained materials such as a ceramic raw material, a binder, and a solvent being insufficiently kneaded, there is caused variance in a strength characteristic and an elastic characteristic of a formed article obtained from the kneaded clay. Therefore, in the preparation of the kneaded clay, it is desirable that the degree of kneading of the contained materials is grasped by an objective index.
The characteristics regarding the transformability, shape-retentivity, and flowability of kneaded clay, that is, so-called rheological properties of kneaded clay significantly influence on the shape-retentivity of the formed article manufactured with the kneaded clay. For example, in the case that a ceramic formed article is manufactured by subjecting kneaded clay to extrusion forming, when flowability of the kneaded clay is poor, the kneaded clay is pressure-bonded to the die for extrusion to generate a cut or a fine on a surface of the resultant formed article. On the other hand, when the flowability of the kneaded clay is good, the formed article manufactured by extrusion forming may easily be deformed.
Thus, though the nature of kneaded clay is important, in an actual production site, the evaluation on the nature of kneaded clay depends on operator's proficient senses of vision, touch, and the like. However, by the evaluation of the nature of kneaded clay due to operator's senses, it is difficult to stably manufacture kneaded clay having high quality. In addition, the acquisition of the skill to recognize good nature of kneaded clay by senses of vision, touch, and the like needs persistent trial and error and training. Even if the skill is acquired through a great deal of effort, it is difficult to hand the skill on to another operator. Therefore, the nature of kneaded clay, which has been recognized as good by an operator's senses, has been tried to grasp by an objective index by a scientific measurement means.
JP-A-9-33420 discloses a method for evaluating a kneaded state of an inorganic powder represented by a ceramic powder and an organic binder by observation with a transmission electron microscope. In this method, by observing the thickness of an organic binder membrane on the surface of the inorganic powder formed by kneading, the degree of kneading of the inorganic powder and an organic binder is evaluated.
JP-A-2000-302525 discloses a method for manufacturing kneaded clay, showing a method for evaluating rheological properties of kneaded clay by an elasticity test and a capillary rheometer test. From the evaluation, it has been found that rheological properties of kneaded clay is significantly influenced by the addition proportion of a binder, a surfactant, and water added to a ceramic raw material.
Nuclear magnetic resonance (NMR) is an indispensable tool for determining a structure of an organic compound as introduced in the “Organic Chemistry” (Volume One), Fourth edition, by K. Peter C. Vollhardt, Neil E. Schore, Chemical Coterie, Chapter 10: Structure Determination by NMR Spectroscopy, Pages 419 to 478, since the state of molecules can be observed based on the energy absorption/ejection phenomenon of nuclear spin. In addition, NMR is used as a tool for grasping the nature of a material containing an organic compound as the main component. For example, JP-A-2001-106728 discloses that a water-absorbing resin excellent in water absorbability can be obtained by specifying the degree of cross-linkage of a resin by the T2 relaxation time of 1H-NMR. Further, Japanese Patent No. 4108648 discloses a method for manufacturing a carbon fiber composite material by the use of the T2 relaxation time of 1H-NMR in order to grasp the state of dispersion of resin in a mixed material. In this method, there is specified a uniform dispersion state of a carbon fiber composite material containing a thermosetting resin, carbon nanofibers, and particles for dispersion.
In the medical diagnosis field, a magnetic resonance imaging method (MRI) where the principle of the NMR is applied has already been used. The MRI uses that the state of water molecules occupying most of a human body is reflected on the proton relaxation time. It is expected that NMR will take one step further from the medical diagnosis field to the production field, in particular, to quality control in a factory or the like.
As a matter of course, the state of clay cannot be grasped accurately even by a method disclosed in JP-A-9-33420 or a method disclosed in JP-A-2000-302525. This is because the senses such as vision and touch of a proficient operator comprehensively catch very complex and miscellaneous factors. In order to grasp the state of clay, it is necessary to decompose the complex and miscellaneous factors, investigate decomposed individual factors based on multilateral indexes, and further investigate the integration of the evaluation data regarding the individual factors. Therefore, still desired are search of unknown factors required for grasping the state of kneaded clay and an evaluation method therefor.
As in JP-A-2001-106728 and Japanese Patent No. 4108648, NMR is a powerful tool in analysis of an organic compound and is widely applied to production of a chemical product containing an organic compound such as resins as the main component. However, there is a barrier against expansion of application to another technical field. For example, it is known that, in accordance with progress of kneading, the binder is sheared to lower the molecular amount and to cause a change in a degree of adhesion of water to the ceramic raw material. Therefore, even if NMR is applied to evaluation of a degree of kneading, enormous knowledge and an operator's proficient skills described above are necessary for selection of a phenomenon measured by NMR and construal of the measurement results.